The present invention provides a working fluid composition for a refrigerating machine containing: a refrigerating machine oil containing, as a base oil, a mixed ester of (A) a complex ester synthesized from a polyhydric alcohol such as neopentyl glycol, a polybasic acid having 6 to 12 carbon atoms, a monohydric alcohol having 4 to 18 carbon atoms and the like, and (B) a polyol ester synthesized from a polyhydric alcohol such as neopentyl glycol and a monocarboxylic fatty acid having 4 to 18 carbon atoms, in a mass ratio of (A) the complex ester/(B) the polyol ester of 5/95 to 95/5; and a refrigerant such as carbon dioxide.